[PROPOSAL] Merge `deepseq-generics` into `deepseq`

**Formal proposal to `libraries@`: http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/23031**

This also replaces the existing `rnf x = seq x ()` default
implementation with a `Generics`-based `DefaultSignature` `rnf` method
implementation

This requires to drop support for GHCs older than GHC 7.2 to avoid
conditional exports due to lack of `Generics` support
Perhaps just complete the sentence with:

> Note: the 'V1' instance is not provided for 'GNFData' in order to trigger a compile-time error, when the user tries to define an instance for an uninhabited type. Values of such types should never be inspected at runtime.

This makes me wonder however. If the user uses generic deriving to define an `NFData` instance for a record type which has one uninhabited field, will we get a compile time error? If so, that sounds a bit inconvenient. I think I'd want the instance to just not force that field instead.